What you'll notice from this concept of probability is that we use words like might and may. Events might happen, but not that they will happen. We can't be certain about the event otherwise the probability for this would be 1.
For example, when we toss a coin in the air we can predict the possibility of it landing head up or tail up, each probability being 0.5 for a 'fair coin'. This says that there is a 50% chance of landing heads and 50% chance of landing tails, but until the coin lands we don't know what it will be.
All probabilities associated with a situation add up to 1. In the tossing of the coin the two possibilities are landing heads or landing tails, or put mathematically the probabilities are added together, thus 0.5 + 0.5 = 1 (excluding extremely remote possibilities, such as landing on edge or being stolen in midair by a bird!).